Electrolytic deposition of platinum,iridium and their alloys

ABSTRACT

1. BATHS, WHICH ARE AQUEOUS AND ACIDIC AND SUBSTANTIALLY BROMIDE-ION FREE, FOR THE DEPOSITION BY ELECTROLYSIS OF PLATINUM AND/OR IRIDIUM, CHARACTERIZED BY THE FACT THAT THEY ARE FORMED OF COMPOUNDS OF PLATINUM AND/OR IRIDIUM GIVING IN AQUEOUS SOLUTION PRIMARILY THE BROMOIRIDIC AND BROMOPLATINIC ANIONS, AND OF AT LEAST ONE ACID SELECTED FROM THE GROUP CONSISTING OF NITRIC, SULFURIC, PERCHLORIC AND BROMIC ACIDS, AND IN WHICH THE PLATINUM AND/OR IRIDIUM CONCENTRATIONS ARE BETWEEN ABOUT 0.1 GRAM AND 60 GRAMS PER LITER AND THE SAID ACID IS PRESENT IN AN AMOUNT OF BETWEEN ABOUT 0.05 AND 1 EQUIVALENT PER LITER OF BATH.

United States Patent C) ELECTROLYTIC DEPOSETION F PLATINUM, IRIDHUM AND THEIR ALLOYS Jean Grosbois, Montreuil-sous-Bois, Jean-Claude Zimmer,

Chatillon-sons-Bagnenx, and Maurice Leroy, St.-Germain-en-Laye, France, assignors to lRhone-Progil, Paris, France No Drawing. Filed Nov. 30, 1972, Ser. No. 310,884 Claims priority, application France, Dec. 2, 1971,

Int. Cl. C2311 5/24, 5/32 US. Cl. 20443 N 14 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates to the electrolytic deposition of platinum, iridium, as well as their alloys, the properties of the thin layers obtained being readily reproducible and such that, in particular when they are deposited on titanium or metals or alloys having a similar anodic behavior, electrodes are obtained which, when used as anodes in electrolysis, have a long life and low overvoltage.

It is known that it is advantageous to use as the anodes in electrolysis metals of the platinum series and more particularly platinum and certain of its alloys. In particular in the electrolysis of alkaline halide solutions, and more particularly of sodium chloride solutions, it has been known for a long time that platinum and even to a greater extent platinum and iridium alloys are highly advantageous for the manufacture of long-lasting anodes. However, from an economical standpoint, these metals are too expensive to use in solid form, and it has also been recommended for some time to use them in thin layer coatings deposited on metal structures which are not subject to attack by the brines used under the electrolysis conditions. Metals suitable for the manufacture of such structures are metals which are covered with films of their oxides under these electrolysis conditions, such as titanium, zirconium, niobium, tantalum, and tungsten, as well as their alloys. These metals are sometimes referred to as film-forming metals.

Such thin layer coatings of platinum, iridium and alloys of these metals can be obtained by various well known methods, among which there may be cited as examples: cathode sputtering, painting with metallizing compositions, and electrolytic coating.

However, the properties of the thin layer coatings obtained by these various methods are not identical with respect to their resistance to chemical, electrochemical and mechanical action, which resistance determines the life of the anodes thus produced, or with regard to the overvoltages with respect to the anodic products produced, which overvoltages must, of course, be as low as possible and must not increase in the course of time.

In particular, the deposition by electrolysis of such thin layer coatingswhich furthermore has great advantages leads only with great difficulty to electrodes of identical 3,841,980 Patented Oct. 15, 1974 and suitable properties as a result of the nature of the electrolysis baths commonly proposed, as Well as their evolution when these baths are to serve for a large number of operations carried out in series. The deposition by electrolysis of platinum, iridium or their alloys is, as a matter of fact, substantially impossible to obtain by means of soluble anodes, and under these conditions, the recharging or reconstitution of the baths with precious metal compounds, added to the natural evolution of these baths and to their evolution as a result of the electrolysis, modifies their composition, preventing any reproducible result both with regard to the rate of obtaining of the metal deposits and their properties.

In a large number of formulas of these baths, the platinum and iridium are introduced in the form of halometallic acids or salts of said acids, or other complex combinations such as those of the metal amines, for instance, to which it has frequently been proposed to add numerous adjuvants, some of which are intended to impart special properties to the thin layer coatings of platinum or iridium obtained. 7

The present applicants have particularly studied the electrolytic deposition of platinum, iridium and alloys of these metals from the corresponding bromometallic acids and salts of said acids in order during the course of manufacture, involving rechargings of baths, to obtain a series of anodes, thin layer coatings of constant, desired properties, and particularly thin layer coatings of alloys of these metals in the desired proportions, and particularly in the proportions previously known to be best. The use of electrolytic baths containing halogens for the depositing of the metals of the platinum series has been known, as a matter of fact, for a long time, and more recently the advantage of using baths containing bromine has been mentioned, particularly with regard to the depositing of iridium. However, the baths proposed are not capable of being used in true industrial fashion as a result of their initial composition and also due to the unfavorable evolution of this composition during use which results in an accumulation of bromide ions and cations, which makes it impossible to obtain thin layer coatings of platinum or iridium and their alloys having the desired properties and to obtain their deposition at constant speed.

It has been found, as a matter of fact, by the applicants that the presence of bromide ions in the baths does not have the same result on the rate of deposition of platinum as on that of iridium. The bromide ion presence, which results either from the intentional addition of hydrobromic acid or of bromide to the baths, or from metal deposition operations, has the effect of decreasing the rate of deposition of the iridium, while the rate of deposition of the platinum is less affected, the effect on the baths containing both metals is intermediate. Accordingly, in the baths proposed by the applicants, it is preferable to maintain at a minimum the quantity of bromide ions present, as these baths should contain furthermore other acids from the group of nitric, sulfuric, perchloric and bromic acids; the perchloric acid being most generally desirable. These acids have little complexing power and their addition permits the electrolytic depositing of the metals, by their acidity avoiding the precipitation in the baths of various insoluble salts.

It has also been found that if one desires to use concentrated baths of metals which permit greater rates of deposition of platinum or iridium, it is then necessary to avoid as much as possible of an accumulation of cations which makes it impossible to maintain these concentra tions.

It is, accordingly, an object of the present invention to provide baths and methods for the deposition of thin layer coatings of platinum or iridium and their alloys which do not encounter the disadvantages of the prior art.

It is another object of the present invention to provide baths and methods for the deposition of thin layer coatings of platinum or iridium and their alloys, which avoid precipitation of insoluble sludges during deposition, wherein an optimum, steady rate of deposition is obtained,

wherein the resulting thin layer coatings are uniform in thickness and resistance to chemical, electrochemical and mechanical action, and which resulting products, when used as anodes, provide low overvoltages.

The foregoing and other objects will be apparent to those skilled in the art from the present description.

GENERAL DESCRIPTION OF THE INVENTION In practice, the platinum and iridium compounds which are easiest to use are bromoiridic and bromoplatinic acids, as well as the ammonium salts of these acids, which salts, however, scarcely make it possible to obtain baths of more than 5 g./ 1. (grams per liter) of the metals. This concentration is sufiicient to obtain a rather high rate of deposition, a concentration of less than 0.1 g./l. of these metals still making it possible to obtain thin layers of good quality with substantial speed. A concentration of 60 g./l. of the metals can be reached by the use of bromoiridic and bromoplatinic acids which can easily be obtained by elimination of the cations from the solutions of salts, followed by distillation. Concentrations reaching such values are, however, not indispensable. In the baths of the invention the concentration of platinum and/or iridium may vary from 0.1 g./l. to 60 g./l.

Accordingly, the baths proposed by the present invention are formed on the basis of compounds which give primarily in aqueous solution brornoiridic and bromoplatinic ions as well as possibly ammonium and the cations of alkaline metals, and one or more of the acids previously mentioned as being of low complexing powder. These acids include nitric, sulfuric, perchloric and bromic acids. Perchloirc acid is preferred. The quantity of these acids added to the baths is preferably between 0.05 and 1 equivalent per liter of bath, values of between 0.1 and 0.5 being particularly well suited, the quantity selected being related to the concentration of metals in the bath. In any event, the amount of acid added should not be too high in order to avoid excessive liberation of hydrogen during the deposition operation.

During repeated operations, the proportions of the various ions and their concentrations are advantageously maintained at the desired values by the periodic or continuous addition of the desired compounds, the elimination of the disturbing ions being effected also periodically or continuously. This elimination is efit'ected in the case of the cations, for instance, in acid exchange resin columns, and the elimination of the bromide ions preferably by a strong bubbling of gas, in practice air, after their oxidation to the state of bromine. This oxidation can be carried out in a circulation outside the platinum and/ or iridium electrolytic deposition cell, by oxygenated water or ozone, for example, by the electrochemical oxidation in an annexed cell on an insoluble anode, or, even better, on the anode of the metal deposition cell itself.

SPECIFIC DESCRIPTION OF INVENTION It goes without saying that the electrolytic baths of the present invention may serve to deposit platinum and/or iridium on any substrate which is a conductor or made conductive on its surface, at times by depositing intermediate layers in order to avoid these substrates being attacked by said baths. The examples which follow have been limited to deposition on titanium due to the great advantage of the anodes which are formed of this metal and covered with nonattackable conductive thin layers, the metals anodically similar to titanium and particularly tantalum giving identical results as to their electrolytic properties in solutions of alkaline halides. Other metals which may be employed as substrates include other filmforming metals, such as zirconium, niobium, tungsten, etc., as recited hereinabove, or their alloys.

In the examples which follow, a number of results are given showing the properties of the titanium electrodes thus obtained in the electrolysis of sodium chloride brines as well as a few results concerning the process itself of the depositing of platinum and/or iridium as a function of the most important factors which have been previously indicated. The final example concerns the manufacture of a series of electrodes by means of a bath maintained at constant composition.

In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by Way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow, and throughout the specification, the quantities of material are expressed in terms of parts by weight, unless otherwise specified.

EXAMPLE 1 The purpose of this example is to show the very strong influence of the concentration of BF ions on the rate of deposition of iridium. For this purpose, a series of titanium electrodes was coated with thin layers of iridium by electrolysis, using a current density of 0.3 amp./dm. (ampere per square decimeter), at a temperature of 75 C., without agitation, from a number of bromoiridic acid solutions containing 4 grams of metal per liter with increasing quantities of hydrobromic acids. To all these solutions perchloric acid was added so as to achieve a concentration of 0.1 N. Table 1 below summarizes the results obtained with respect to the rate of deposition, that is to say, the thickness in microns of metal deposited per hour, as a function of the Br/ Ir ratio, that is to say, the ratio of the number of Branions coming from the hydrobromic acid added, to the number of iridium atoms present.

TABLE 1 Rate of deposition Ratio Br/Ir: in micron/hr.

This table shows that for a ratio of Br/Ir of 36, the depositing of iridium cannot be obtained under the conditions of the experiment.

Under similar conditions in which the iridium is re placed by platinum, the rate of deposition of the platinum is only very slightly afiected by the variation of the concentration of Br ions; this rate, as a matter of fact, tends to decrease substantially only for B1 /Pi. ratios of the order of 10 EXAMPLE 2 This example is intended to show the action of an acid of low complexing power, such as perchloric acid. There is used a bath which is employed in conventional fashion for the electrolytic deposition of iridium and is formed of a solution of bromoiridic acid of 2 g./l. (grams per liter) of iridium. Such a bath maintained at a temperature of 75 C. for 2 hours permitted the deposition of an amount of sludge containing about one-sixth of the iridium present. This drawback is entirely eliminated if perchloric acid is added in such a manner as to obtain a concentration of 0.1 N in the bath.

The same result is obtained by the addition of the other acids mentioned, that is to say, sulfuric, nitric and bromic acids, in the same concentration.

EXAMPLE 3 This example describes the manufacture of a titanium electrode coated with iridium and its use as an anode.

There is employed a bath which contains 11 g. per liter of ammonium bromoiridate, (NH IrBr and perchloric acid in a concentration of 0.14 N. This bath is maintained at 7 5 C. and the electrolysis is carried out between an insoluble anode formed of titanium coated with iridium and a cathode which is to be coated with iridium and is formed of a sanded plate of titanium. The current density employed is 0.3 amp./dm. At the end of one hour there is obtained a thin layer of good adherence of a thickness of 1.5 microns. The electrode iridiated in this manner is used as anode in an electrolysis test cell for the production of chlorine from a sodium chloride brine of 300 g./l. maintained at 80 C.; under these conditions, the potential of this electrode with reference to a calomel electrode is 1.12 volts for a current density of 30 amp./dm.

EXAMPLE 4 This example is intended to show the stability of the baths in accordance with the invention. The bath tested is similar to the bath of Example 3 and differs only by the amount of iridium present therein which corresponds here to 7.35 grams per liter of ammonium bromoiridate. This bath is maintained for 2 hours at 75 C. and does not form any sludge. A deposit of a thickness of 0.95 micron is carried out with this bath in the same general fashion as in Example 3, giving, in the electrolysis of sodium chloride solutions, results similar to those obtained with the deposition of the preceding example.

EXAMPLE 5 This example concerns a platinizing bath which contains 0.75 grams per liter of ammonium bromoplatinate and the concentration of which is 0.35 N in perchloric acid. On a sanded titanium cathode there is obtained, with this bath maintained in agitation, a thin layer of platinum of a thickness of 0.45 micron Within 15 minutes, with a current density of 0.35 amp./dm. This electrode, used as an anode for the production of chlorine from a brine containing 300 g./l. of sodium chloride and a pH of 4, maintained at 90 C., shows a potential of 1.09 volts referred to a calomel electrode for a current density of 100 amp./ dmf".

EXAMPLE 6 This example describes the production of a series of mixed platinum/iridium thin layer coatings in which these metals are present in different proportions. Two master solutions of platinum and iridium are prepared by separately dissolving 1.5 grams of ammonium bromoplatinate in one liter of water and 1.1 grams of ammonium bromoiridate in one liter of water. By means of these stock solutions there are then prepared five baths of different Pt and Ir contents, and perchloric acid is added thereto so as to obtain a concentration of 0.2 N in said acid. Thereupon one proceeds by electrolysis, with agitation, using these solutions maintained at a temperature of 75 C., with the deposition of metal layers on 5 identical sanded titanium electrodes used as cathodes, the potential of these elec trodes being '0.2 volt, referred to a calomel electrode. In the following Table 2 there are set forth the amounts of iridium and platinum in the baths, the thickness of the deposits obtained, the proportion of iridium in the deposits and the potentials of these electrodes, referred to a calomel electrode, when they are used as anodes in a brine electrolysis cell containing 300 g./l. NaCl, the current density being 100 amp/dmf".

TABLE 2 Amount of Thick- Percent- Potential Iridium Platinum ness of the age of referred in the in the deposits iridium to the Elecbaths in, baths in, in, in the calomel trodes g./l. g./l microns deposits electrode The potentials of these electrodes, referred to a calomel electrode, show that the o'vervoltages are very low and that these different electrodes are therefore well suited for the electrolysis of sodium chloride brines.

EXAMPLE 7 This example concerns the continuous manufacture of a series of 12 electrodes of industrial dimensions intended for the electrolysis of sodium chloride brines so as to obtain chlorine and for which it is desired to obtain thin metal layers in which the proportion of iridium is substantially 20%. One starts by preparing separate concentrated solutions of platinum and iridium in the following manner:

Solution A of platinum.-440 grams of ammonium bromoplatinate are dissolved in 20 liters of water in the presence of a strong cationic resin in acid form, whereupon this solution is passed over a column of the same resin so as to assure an elimination which is as complete as possible of the NH ions. After rinsing the ion exchange resin column, there are finally obtained 23 liters of solution of 5.2 g./l. of platinum which is concentrated by distillation under reduced pressure, to 60 g./l. of platinum.

Solution B of iridium-In the same manner as in the case of Solution A, 3 liters of a solution of 10.5 g./1. of iridium are prepared from grams of ammonium bromoiridate by dissolving in water, followed by exchange of the NH,+ ions which is then concentrated to 33 g./l. of iridium.

The initial solution used for the electrolysis is obtained by bringing to 10 liters the mixture of 100 ml. of Solution A, ml. of Solution B, and 100 ml. of a 10 N solution of perchloric acid. The Ir/ Pt ratio of this solution of 0.7 which corresponds substantially under the electrolysis conditions used to a proportion of 20% iridium in the metallic thin layers obtained.

Each deposition is carried out at a temperature of 75 C. with a strong bubbling of air assuring the entrainment of the bromine formed on the anode by oxidation of the Br ions, as well as the agitation and homogenization of the baths. The current density is 1.4 amp./dm. The electrolysis times are about 5 minutes, which makes it possible to obtain platinum/iridium thickness of about 0.5 micron corresponding to about 10 grams.

After each operation, this amount of deposited platihum/iridium as well as the water lost by evaporation is substantially compensated for by adding 200 ml. of a solution resulting from the mixing of 1850 ml. of Solution A with 850 ml. of Solution B. The iridium content of the thin layers obtained is sufficiently uniform and varies from about 18 to 21%; the anodes thus obtained give satisfaction in an industrial diaphragm electrolysis cell intended for the production of chlorine and caustic soda solution.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. Baths, which are aqueous and acidic and substantially bromide-ion free, for the deposition by electrolysis of platinum and/ or iridium, characterized by the fact that they are formed of compounds of platinum and/or iridium giving in aqueous solution primarily the bromoiridic and bromoplatinic anions, and of at least one acid selected from the group consisting of nitric, sulfuric, perchloric and bromic acids, and in which the platinum and/or iridium concentrations are between about 0.1 gram and 60 grams per liter and the said acid is present in an amount of between about 0.05 and 1 equivalent per liter of bath.

2. Baths according to Claim 1, also containing a member selected from the class consisting of ammonium ions and cations of the alkaline metals.

3. Baths according to Claim 1, wherein the amount of said acids is between about 0.1 and 0.5 equivalent per liter of bath.

4. Processes for depositing platinum and/or iriduim by electrolysis by means of a bath according to Claim '3.

5. Processes for depositing platinum and/or iridium by electrolysis by means of a bath according to Claim 2.

6. Processes for depositing platinum and/or iridium by electrolysis by means of a bath according to Claim 1.

7. Processes according to Claim 6, characterized by the fact that bromide ions are removed at least intermittently.

8. Processes according to Claim 7, characterized by the fact that the removal of the bromide ions is effected by chemical or electrochemical oxidation.

9. Processes according to Claim 7, characterized by the fact that the removal of the bromide ions is effected by bubbling of gas.

10. Processes according to Claim 6' wherein said bath is intermittently purified to remove interfering ions.

11. Processes according to Claim 10, characterized by the fact that the removal of the ions is effected by an ion exchange resin.

12. Processes according to Claim 6, wherein said bath is continuously purified to remove interfering ions.

13. Processes according to Claim 12, characterized by the fact that the removal of the ions is elfected by an ion exchange resin.

14. Processes according to Claim 6, characterized by the fact that bromide ions are removed continuously.

References Cited UNITED STATES PATENTS OTHER REFERENCES A. Kenneth Graham: Handbook, p. 344 (1962).

Electroplating Engineering GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 204-47 

1. BATHS, WHICH ARE AQUEOUS AND ACIDIC AND SUBSTANTIALLY BROMIDE-ION FREE, FOR THE DEPOSITION BY ELECTROLYSIS OF PLATINUM AND/OR IRIDIUM, CHARACTERIZED BY THE FACT THAT THEY ARE FORMED OF COMPOUNDS OF PLATINUM AND/OR IRIDIUM GIVING IN AQUEOUS SOLUTION PRIMARILY THE BROMOIRIDIC AND BROMOPLATINIC ANIONS, AND OF AT LEAST ONE ACID SELECTED FROM THE GROUP CONSISTING OF NITRIC, SULFURIC, PERCHLORIC AND BROMIC ACIDS, AND IN WHICH THE PLATINUM AND/OR IRIDIUM CONCENTRATIONS ARE BETWEEN ABOUT 0.1 GRAM AND 60 GRAMS PER LITER AND THE SAID ACID IS PRESENT IN AN AMOUNT OF BETWEEN ABOUT 0.05 AND 1 EQUIVALENT PER LITER OF BATH. 